class Watcher {
    constructor(vm, key, cb) {
        this.vm = vm
        this.key = key
        this.cb = cb

        Dep.target = this

        this.oldValue = vm[key]

        Dep.target = null
    }

    update() {
        let newValue = this.vm[this.key]
        if (this.oldValue === newValue) return
        
        this.cb(newValue)
    }
}